In image forming apparatuses that form color images by electrophotographic or inkjet printing methods, during image formation the amount of toner or ink is adjusted in response to the image signal. In an apparatus of this type, a toner/ink consumption reduction mode is often provided for reducing the amount of ink or toner consumed and thereby reduce running costs. Moreover, when C, M, Y and K (CMYK) signals are input from an external source and an image is output, it is sometimes necessary to place a limit on the amount of toner or ink adhered to a unit of surface area of a recording medium, depending on the characteristics of the image forming portion of the printer, the characteristics of the toner or ink and the processing method used for further increasing printing speed. In such cases, a process is carried out to lower the overall signal level of the image signals to a constant limiting value so as to keep the amount of toner or ink consumed within the capabilities of the printer. In such a processing method, it is known that the CMYK image signal levels are reduced to a certain constant proportion, as described, for example, in Japanese Laid-Open Patent Publication No. 9-37094, Japanese Laid-Open Patent Publication No. 10-73978 and U.S. Pat. No. 6,023,344.
Specifically, in the example of a color copier, when converting from R, G and B (RGB) information or other such luminance information input from a scanner to CMYK density information, a conversion table is designed so that the post-conversion density information does not exceed a limiting value.
In a color printer that generates print images using luminance information such as RGB, similarly to the color copier, a conversion table for converting luminance information to CMYK density information is designed so that post-conversion density information does not exceed a limiting value, so as to control the amount of toner or ink consumed.
A conventional color printer generates print images using CMYK density information. Luminance information (RGB information) is converted to CMYK density information for each of the characters and photographic objects, comprised in the print image. While carrying out such conversions, the conversion table used when converting from the luminance information to the CMYK density information is designed so that the converted CMYK density information does not exceed a limiting value, so as to control the amount of toner or ink consumed.
With a color composite machine, because the scanned image and the generated print data are stored on a hard disk as a mail box function and sent to a host PC via a network as a transmission function, the image data is generated in dot sequence. When print image data is generated as a color printer function, there is a CMYK mode that generates print image data using density information CMYK and a RGB mode that generates print image data using luminance information such as RGB. When the print image data is generated using CMYK density information, as with the color printer described above, when converting each graphic object to CMYK information, the conversion table is designed so that the post-conversion density information does not exceed a limiting value, so as to control the amount of toner or ink consumed.
In the conventional generation of print image data using CMYK density information, as described above, the CMYK print information of each graphic object is converted to CMYK density information, and the print image data is generated using that graphic object.
In the generation of print image data, when performing logical operations between a plurality of objects and generating pixel data for printing, an overprint function, which adds object color to a background object, and a trapping function, which expands the objects slightly in order to correct a color blur due to the electrophotographic process and slightly overlaps the edges of different colors, are sometimes used. In these cases, even though the amount of toner or ink consumed by each individual graphic object might be within the limiting value, there are nevertheless pixel data which exceed these limiting values in the print image data, which then lead to excess toner or ink consumption and cause the toner or ink device to deteriorate.
Further, when a process of reducing the amount of toner or ink, as described above, is carried out, the quality of the image declines, such as fainting of with the overall color of the printed matter output, and greater color changes than anticipated, and so forth.